1. Field of the Invention
The present invention relates, in general, to a process for preparing a metal oxide slurry useful in semiconductor chemical mechanical polishing (CMP) and, more particularly, to use of the counter collision of a metal oxide slurry by injecting it from two orifices which are designed to face each other at a high speed, whereby the metal oxide slurry can be allowed to be narrow in particle size distribution and superior in dispersion stability and polishing rate in addition to showing an exceptionally decreased xcexc-scratch frequency.
2. Description of the Prior Art
A CMP process, a kind of lithography, is utilized in fabricating semiconductors. As semiconductors become miniaturized with denser populations to more complex multilayer structures, planarization, which can be attained by the CMP process, is indispensable for the high integration of semiconductors.
To be useful in the CMP process, generally, metal oxide slurry is required to show excellent dispersibility and polishing rate as well as to leave as few defects, such as xcexc-scratches, after polishing, as possible, in addition to being high in purity.
All of these requirements, except for purity, are closely concerned with the particle size and distribution of the metal oxide. In regard to the particle size, smaller particles are preferable because they show better dispersion stability and provide fewer xcexc-scratches, but are disadvantageous in polishing efficiency because the smaller the particles are, the slower the polishing rate is. In an aspect of particle size distribution, of course, the particles are preferably distributed in a narrow size range. In other words, the particles more uniform in size bring about better polishing results. For example, when using slurry with a broad range of particle sizes, the abraded surface is poor in flatness and has a significant amount of xcexc-scratches thereon.
Thus, when selecting the particle size and size distribution of slurry for CMP, account must be taken of polishing rate, dispersion stability and xcexc-scratch frequency.
U.S. Pat. No. 5,382,272 discloses a preparation process of polishing compositions which exhibit high polishing rates. The compositions are based on SiO2 and used to polish Si wafers. The compositions are prepared by blending silica and deionized water in a high speed mixer and agitating them in an agitator mill which contains a mill medium (bead). It is written that the base abrasive is activated by addition of a second cation, such as Ce4+ and Zr4+, so that the polishing rate is enhanced. The process disclosed in this patent suffers from disadvantages. The beads are inevitably polluted during the dispersion which is accomplished by the collision between the abrasive and the bead. In addition, a tailing phenomenon occurs, making it difficult to produce a slurry whose particles are in a narrow size distribution. Further, the beads are etched during milling, so that their dispersing ability is decreased. In fact, because the slurries produced are significantly different from each other in particle size and size distribution, a constant polishing ability cannot be expected from the slurries.
In another known preparation process, a fluid is rotated at a high speed by a rotor sold by IKA, Germany and collided with a stator. This technique, even if enhanced relative to the process of U.S. Pat. No. 5,382,272, has a problem in that the stator is etched as a result of the wall collision so that a significant decrease in dispersion capability results.
These conventional techniques all are known to produce particles which have a size of xcexcm. They are too large to be used in CMP. In particular, they cannot be used as a CMP slurry for shallow trench isolation because xcexc-scratches, if occurring during the isolation process, cause fatal damages to the function and yield of semiconductor devices.
Another technique relating to a CMP slurry is disclosed in WO Pat. No. 9 747 430. With use in polishing Si wafers, the slurry composition of this patent contains SiO2 as an abrasive, monoethanol amine as a pH-adjusting agent, and additives such as NH4+, Cs+ and Ba3+. The pH-adjusting agent excludes the plausible possibility that conventional pH-adjusting agents, such as KOH or NH4OH, may diffuse into wafers during a polishing procedure and act as a pollutant. However, this slurry exhibits a relatively slow polishing rate ranging from 1,500 to 2,500 A/min. In addition, nowhere is mentioned a dispersion process for CMP slurry.
U.S. Pat. No. 5,342,609 described a method and apparatus for forming emulsions, in which collisions between oils, cavitation, and shearing stresses are utilized, in combination. The apparatus, called a microfluidizer, is known to be applied for various purposes and superior in emulsification. However, it has not yet been applied for the dispersion of particles, such as metal oxides.
An example of the prior art using the microfluidizer is found in U.S. Pat. No. 5,342,609. According to the patent, a particle mixture comprising calcium and oxianions is dispersed in a microfluidizer, so as to produce a composition with a particle size of 5 nm. However, this composition is used for diagnostic purposes, such as MRI, X-ray and ultrasound, but not for semiconductor polishing.
Therefore, it is an object of the present invention to overcome the problems encountered in prior arts and to provide a process for preparing a metal oxide slurry useful in semiconductor chemical mechanical polishing (CMP), whereby the metal oxide slurry, which is dispersed as a result of the complex occurrence of counter collisions, wall collisions and cavitation, can be allowed to be narrow in particle size distribution and superior in dispersion stability and polishing rate in addition to showing an exceptionally decreased xcexc-scratch frequency.
In accordance with the present invention, the above object could be accomplished by a provision of a process for preparing a metal oxide CMP slurry suitable for semiconductor devices, wherein a mixture comprising 1 to 50 weight % of a metal oxide and 50 to 99 weight % of water is mixed in a pre-mixing tank, transferred to a dispersion chamber with the aid of a transfer pump, allowed to have a flow rate of not less than 100 m/sec by pressurization at 50 atm with a high pressure pump, and subjected to counter collision for dispersion through two orifices which are designed to face each other in the dispersion chamber.